Appears to happen if you break their workstation. The issue resolves itself after about 10 - 15 minutes if you make the applicable work station available again.
Just created new villagers in 1.14.3 with this issue occurring after several trades of all levels. Of 12 villagers just created. 4 of them have this issue. One is a novice with two or three trades, the others are Expert. All librarians.
Steve Smallwood
Appears to happen if you break their workstation. The issue resolves itself after about 10 - 15 minutes if you make the applicable work station available again.